This invention relates generally to biocompatible polymer compositions that rapidly crosslink to form a gel. More particularly, the invention relates to a composition prepared by admixture of individually reactive polymer components, wherein the admixture initiates rapid crosslinking and gel formation. Such compositions are particularly well suited for use in a variety of tissue-related applications in which rapid adhesion to the tissue and gel formation is desired. Accordingly, the invention additionally relates to methods of using the compositions as bioadhesives, for tissue augmentation, in the prevention of surgical adhesions, for coating surfaces of synthetic implants, as drug delivery matrices, for ophthalmic applications, and in other applications, as discussed herein and/or as appreciated by one of ordinary skill in the art.
The use of polymer compositions in tissue engineering is now widely recognized, particularly those compositions manufactured with synthetic polymers. In contrast to many naturally derived compositions, synthetic polymer compositions can be formulated to exhibit predetermined physical characteristics, such as gel strength, as well as biological characteristics, such as biodegradability.
In a variety of tissue engineering applications, it is desirable to use compositions that can be administered as liquids, but which subsequently form gels at the site of administration. Such in situ gel-forming compositions are convenient to use since they can be administered as liquids from a variety of different devices, and are adaptable for administration to any site, since they are not preformed. Many different mechanisms have been described that can be used to promote gel formation in situ. For example, photoactivatable mixtures of water-soluble co-polyester prepolymers and polyethylene glycol have been described as useful in the preparation of gel barriers and drug release matrices. In another approach, block copolymers of a Pluronic(trademark) poloxamer have been designed that are soluble in cold water, but form insoluble gels that adhere to tissues at body temperature (Leach et al. (1990) Am. J. Obstet. Gynecol. 162: 1317-1319 (1990)). Polymerizable cyanoacrylates have also been described for use as tissue adhesives (Ellis, et al. (1990) J. Otolaryngol. 19:68-72 (1990). In yet another approach, two-part synthetic polymer compositions have been described that, when mixed together, form covalent bonds with one another, as well as with exposed tissue surfaces. (PCT WO 97/22371, which corresponds to U.S. application Ser. No. 08/769,806.) In a similar approach involving a two-part composition, a mixture of a protein and a bifunctional crosslinking agent has been described for use as a tissue adhesive (U.S. Pat. No. 5,583,114.) One difficulty encountered when designing in situ gel forming compositions is that optimizing the composition to enhance gel formation may worsen tissue inflammation at the site of administration. A possible explanation for this effect is that highly reactive composition components that are capable of rapid gel formation may adversely affect tissue surfaces.
The compositions of the present invention have been formulated to provide for rapid gelation, while decreasing the likelihood and/or severity of tissue inflammation at the site of administration relative to that associated with the previously described compositions.
Accordingly, in one aspect of the invention, a reactive polymer composition is provided that comprises an admixture of two or more biocompatible, reactive components selected so as to rapidly react with each other to form a crosslinked gel. The first component, component xe2x80x9cA,xe2x80x9d is a sulfhydryl-containing component having m sulfhydryl groups, and the second component, component xe2x80x9cB,xe2x80x9d is a sulfhydryl-reactive component B having n sulfhydryl-reactive groups capable of reaction with the m sulfhydryl groups to form covalent bonds, wherein mxe2x89xa72 and nxe2x89xa72, and generally the sum of m+nxe2x89xa74. Preferably, at least one of m and nxe2x89xa73, and more preferably, m and n are each xe2x89xa74; in this way, sufficient reactivity for rapid formation of a three-dimensional polymeric gel is ensured. For extremely fast-reacting compositions, both m and n are each xe2x89xa712. The compositions may be used either in situ or ex situ, to give a biocompatible crosslinked gel having utility in a host of different contexts, e.g., in bioadhesion, biologically active agent delivery, tissue augmentation, and other applications. The preferred context, however, involves crosslinking and gelation in situ.
The reaction conditions necessary for the rapid crosslinking reaction to take place will depend on the particular components A and B. When neither component is a liquid at room temperature, the reaction must be carried out in an added solvent, preferably a sterile aqueous medium. If at least one of the components is a liquid and capable of serving as a reaction solvent, the reaction may be conducted xe2x80x9cneatxe2x80x9d (also referred to as xe2x80x9cin bulkxe2x80x9d), i.e., no added solvent is necessary. In addition, for components that crosslink via a nucleophilic substitution mechanism, such that covalent bonds are formed by nucleophilic attack of the sulfhydryl groups on electrophilic sulfhydryl-reactive groups, an added base is typically necessary to increase the nucleophilicity of the sulfhydryl groups such that the crosslinking reaction occurs sufficiently rapidly. For components that crosslink via other mechanisms, an added base is generally not required (although one may be present). For example, reaction of unconjugated olefins with sulfhydryl groups does not involve nucleophilic substitution, but rather requires light or other radiation effective to generate the sulhydryl radical Rxe2x80x94Sxe2x80x94, and a suitable free radical initiator.
The components of the composition will generally be admixed, under the reaction conditions appropriate to promote rapid crosslinking of the selected components, e.g., in bulk, in an aqueous liquid, with added base, and/or in the presence of radiation and/or a free radical initiator), immediately prior to administration. Alternatively, the components may be individually applied to the site of administration under appropriate reaction conditions, such that admixture occurs at the administration site.
It will be appreciated that more than one sulfhydryl-containing component and/or more than one sulfhydryl-reactive component may be present in the reactive composition.
At least one of the reactive components A and B is a polymer, preferably a hydrophilic polymer, and may be naturally occurring, purely synthetic, or semisynthetic polymer, wherein xe2x80x9csemi-syntheticxe2x80x9d refers to a chemically modified naturally occurring polymer. The non-reactive portion of the polymer is referred to as its xe2x80x9ccore,xe2x80x9d with either sulhydryl groups or sulfhydryl-reactive groups bound thereto. Suitable polymer cores include synthetic polymers, as noted above, polyamino acids, polysaccharides, and the like. The molecular weight of the polymer can vary depending on the desired application. In most instances, the weight average molecular weight is about 100 to about 2,000,000, preferably about 1,000 to 1,000,000, more preferably about 1,000 to about 100,000, and most preferably about 1,000 to about 20,000. When the polymer core is polyethylene glycol, the preferred molecular weight is in the range of about 1000 to about 20,000, optimally about 10,000.
One or more of the reactive components in the composition may be a low molecular weight crosslinking agent, although it is preferred that not more than one of the components is comprised of such an agent. Typical low molecular weight crosslinking agents are comprised of a hydrocarbyl moiety containing 2 to 14 carbon atoms and at least two functional groups, i.e., sulfhydryl groups or sulfhydryl-reactive groups. Generally, although not necessarily, any low molecular weight component that is employed serves as the sulfhydryl-reactive component rather than as the sulfhydryl-containing component, and is used in conjunction with a thiolated polymer.
The sulfhydryl groups and the sulfhydryl-reactive groups may be directly bound to the component, indirectly bound to the component through a linking group, or indirectly bound through an extended linking moiety termed a xe2x80x9cchain extender.xe2x80x9d Chain extenders can activate or suppress reactivity of the functional groups, and can also be used to provide sites for hydrolysis or degradation. Suitable chain extenders include poly(amino acids), poly(lactones), poly(anhydrides), poly(orthoesters), poly(orthocarbonates), poly(phosphoesters), poly(alkylene oxides) and enzymatically cleavable peptide groups.
The compositions of the present invention form gels with gel times of less than 1 minute, preferably less than 30 seconds, and most preferably less than 15 seconds. The strength (i. e., elastic modulus or Gxe2x80x2) of the resultant gels depends on the application for which the composition is adapted, but is generally in the range of about 1 N/cm2 to about 100 N/cm2, preferably in the range of about 1 N/cm2 to 20 N/cm2 for a soft gel, or in the range of about 40 N/cm2 to about 100 N/cm2 for a harder gel.
In addition to the reactive components, the reactive compositions of the invention may include additional materials as well, such as glycosaminoglycans, proteins such as collagen, nucleotidic materials such as DNA, cells, hemostatic agents, genes, therapeutic agents, antibiotics, growth factors, and the like.
When the sulfhydryl-reactive component is such that a base is required for the reaction between the sulfhydryl and sulfhydryl-reactive groups to occur, the components of the composition will generally be admixed in an aqueous medium having a pH in the range of about 7.5 to about 11, immediately prior to administration. Alternatively, as above, the components may be individually applied to the site of administration, such that admixture occurs at the administration site, providing that the admixture is in an aqueous medium having a pH in the range of about 7.5 to 11. It is also possible to apply such components in premixed but inactive formxe2x80x94i.e., in an acidic aqueous mediumxe2x80x94and then activate them, with base, either at the site of administration or immediately before application. Preferred bases are generally, although not necessarily, non-nucleophilic.
Analogously, when the sulfhydryl-reactive component is such that the crosslinking reaction requires light or other radiation and a free radical initiator, the components may be activated, i.e., with light or other suitable radiation, either immediately prior to or following administration.
In another aspect of the invention, then, a method is provided for the formation of a biocompatible crosslinked gel in situ, wherein the method comprises:
(a) admixing (i) a biocompatible crosslinking component A having m sulfhydryl groups wherein mxe2x89xa72, (ii) a biocompatible crosslinking component B having n sulfhydryl-reactive groups, wherein nxe2x89xa72, capable of undergoing a nucleophilic substitution reaction with the m sulfhydryl groups upon admixture of components A and B so as to form a gel in less than one minute, and optionally (iii) at least one pH-adjusting agent, to provide a reactive composition, wherein the at least one pH-adjusting agent, if used, provides the reactive composition with a pH in the range of about 7.5 to about 11; and
(b) allowing the components to react, so as to crosslink and form a gel.
Unless one or both components are liquids at the reaction temperatures employed (generally ambient temperature up to body temperature), the reaction is carried out in a solvent, preferably a sterile aqueous medium, in which case the aforementioned method further includes admixing (i), (ii) and optionally (iii) with (iv) a solvent.
In a related embodiment, wherein the components are contained in an inactive form and then activated with base prior to use, the method comprises:
(a) providing, in an aqueous medium having a pH in the range of about 3 to 6, a biocompatible crosslinking component A having m sulfhydryl groups, wherein mxe2x89xa72, and a biocompatible crosslinking component B having n sulfhydrl-reactive groups wherein nxe2x89xa72, and further wherein the sulfhydryl-reactive groups are capable of undergoing a nucleophilic substitution reaction with the m sulfhydryl groups upon admixture of components A and B in a basic aqueous medium, so as to form a gel in less than one minute;
(b) increasing the pH of the aqueous medium to a pH in the range of about 7.5 to about 11, by adding at least one basic reagent to the aqueous medium; and
(c) allowing the components to react, so as to crosslink and form a gel.
In still another aspect of the invention, a method is provided for the formulation of a biocompatible crosslinked gel in situ, wherein the method comprises:
(a) admixing (i) a biocompatible crosslinking component A having m sulfhydryl groups wherein mxe2x89xa72, (ii) a biocompatible crosslinking component B having n sulfhydryl-reactive groups, wherein nxe2x89xa72, capable of undergoing a free radical coupling reaction with the m sulfhydryl groups upon admixture of components A and B in the presence of light or other radiation, and (iii) a free radical initiator;
(b) irradiating the admixture prepared in (a); and
(c) allowing the components to react, so as to crosslink and form a gel.
Again, unless one or both components are liquids at the reaction temperatures employed (generally ambient temperature up to about body temperature), the reaction is carried out in a solvent, preferably a sterile aqueous medium, in which case the aforementioned method further includes admixing (i) and (ii) with (iii) such a solvent.
In another embodiment, a gel-forming system is provided in which the components are not admixed, but are physically separated. For example, the gel-forming system may be comprised of:
(a) a biocompatible crosslinking component A having m sulfhydryl groups wherein mxe2x89xa72, in a liquid medium having an alkaline pH; and
(b) a biocompatible crosslinking component B having n sulfhydryl-reactive groups, wherein nxe2x89xa72, in either a liquid medium having a neutral or acidic pH or in powder form, and further wherein the sulfhydryl-reactive groups are capable of undergoing a nucleophilic substitution reaction with the m sulfhydryl groups upon admixture of components A and B in the presence of base.
In other aspects of the invention, methods of using the compositions encompassed by the present invention are provided, including drug delivery methods, bioadhesion, delivery of cells and genes, tissue augmentation, prevention of adhesions following surgery or injury, and implant coating. Other methods of use are also within the scope of the invention, as will be described below.